It is known to coat metal sheet or strip with thermoplastic resin on one or both sides to improve the corrosion resistance, formability, appearance or other properties of the material. The coating can be applied by a variety of processes such as roll coating, reverse roll coating, spraying, electrocoating, powder coating, and lamination. The coated strip may be used for applications such as in cans and can ends, foil pouches, lidding stock, appliances, electrical devices, construction, aerospace or automotive body sheet.
U.S. Pat. No. 5,093,208 to Heyes et al. discloses a method for forming a laminated metal sheet in which a precast thermoplastic polyester film is pressed against one or both surfaces of a metal sheet to adhere the film to the sheet in a non-crystalline form. The uncoated sheet of metal is heated to a temperature above the melting point of the polyester film and the film is applied to the sheet under pressure to form a laminate material. The laminate material is then heated to above the melting point of the film to improve the bond of the plastic film to the metal and is quenched rapidly to a temperature below the glass transition point of the polyester to form a non-crystalline polyester. The quenching is done by passing the laminate through a curtain of water.
European Patent Application 0067060 in the name of Taiyo Steel Ltd. discloses a method of producing a coated metal plate by directly extruding a thermoplastic resin onto the heated surface of the plate. According to that patent application, molten resin is applied directly from the extrusion die to the metal plate without forming the resin into an independent film. The thickness of the film can be less than 50 microns and preferably down to 35 to 5 microns. The patent states that since the step of forming an independent film is omitted, the cost of producing the coated metal is reduced. Suitable thermoplastic resins used for coating of metal surfaces include polyolefins, acrylic resins, polyesters, polyamides, polyvinylchlorides and many other resins as listed in the published patent application. The resin can be coated either as a mono-layer or multilayers of the same or different resins. The patent application discloses applying the resin on only one side of the metal strip.
An improved process is desired for applying a thin polymer coating on both sides of a metal strip suitable for use in applications such as packaging. A process is desired for producing tight adhesion or welding of the polymer to the strip so that the polymer will not delaminate during subsequent forming of the strip or use of the products produced from the strip.